1. Field of the Invention
The present invention relates to a resonator and a band-pass filter using the same, and particularly, it relates to a resonator used at a band of several GHz and a band-pass filter using the same.
2. Description of the Prior Art
In a conventional resonator, there are strip line resonators of 1/2 wavelength as shown in FIGS. 20 and 21. The resonator 1 has a line electrode 3 whose both ends are open on one surface of a dielectric plate 2, and has an earth electrode 3 formed throughout the other surface of the dielectric plate 2. In the resonator 1, when .lambda. is defined as wavelength and .epsilon. is defined as an effective dielectric constant of the dielectric plate 2, the length L.sub.1 of the line electrode 3 is determined as shown in Equation 1. ##EQU1##
As shown in FIG. 22, there is a resonator of 1/4 wavelength having a line electrode 3 whose one end is connected to an earth electrode via a side face of the dielectric plate 2. The length L.sub.2 of the line electrode 3 of the resonator 1 is determined as shown in Equation 2. ##EQU2##
As shown in FIG. 23, there is a resonator having a circinate coil electrode 4 which is formed on one surface of dielectric plate 2. In the resonator 1, an earth electrode 5 is formed on the other surface of the dielectric plate 4 so that it is opposed to the coil electrode 4. An earth terminal 6 is drawn out from one end of the coil electrode 4 to connect to the earth electrode 5, and a an input/output terminal 7 is formed at a distance from the earth terminal 6. In the resonator 1, since the coil electrode 4 is formed in a circinate shape, the resonator 1 can be miniaturized even when the coil electrode 4 is long. And, a band-pass filter can be obtained by locating resonators side by side and magnetically coupling the resonators with each other.
However, in the resonator of 1/2 wavelength or the resonator of 1/4 wavelength, the resonator having a resonance frequency of 2-3 GHz becomes substantially large because the line electrode is long. In the resonator having a coil electrode, since the coil electrode has a circinate shape, magnetic fluxes are influenced by one another between the adjacent lines. Thus, it is difficult for an electric current to flow. Therefore, a substantial resistance increases and Q becomes lower. When the resonator is miniaturized, the distance between the adjacent lines becomes short, and influence of magnetic fluxes becomes larger, thus the current flow problem increases. Because of the current flow problem, the insertion loss of the band-pass filter using such resonators increases.